December 4, 2012 AT 4:58 pm

How-to: Make a Low Power Coin Cell Voltage Logger

In developing our new TIMESQUARE watch, we knew that power use would be a hairy issue. The entire circuit, including an ATmega328P microcontroller and an 8×8 LED matrix, is powered from a single CR2032 lithium coin cell. We obsessed over different LED multiplexing arrangements and processor sleep modes, always trying to trim the power draw just a little bit more.

With the right tools such as the EEVblog μCurrent and a good multimeter, measuring the most minute current changes is a simple task. But translating this into battery longevity isn’t so cut-and-dried…the stated capacity in the battery datasheet assumes a small and constant load, while the watch current can vary greatly. What’s more, the relationship between current draw and battery longevity isn’t necessarily linear. This gets messy. Sometimes you just need to put math and theory aside, plug the thing in and observe the actual outcome.

To that end, we built a test fixture to simulate a consistent use case: activating the watch display once per minute and monitoring the battery voltage as it declines, allowing us to objectively compare different versions of the watch software. The raw data is logged to an SD card for later review and conversion into nice graphs. So this is primarily a tutorial on using the Data Logging Shield for Arduino, but along the way there are some good ancillary tidbits on hardware and software.